Process and apparatus are disclosed for forming a layer of a stoichiometric metal compound on a semiconductor wafer by reactive sputtering a metal target in a chamber in the presence of a reactive gas, wherein the negative potential on a metal target is increased or decreased to change the supply of sputtered metal atoms available to react with the atoms of the reactive gas at a fixed flow of the gas by resetting the power level of a constant power source electrically connected to the target and a path is provided for the flow of reactive gas to the zone between the target and the wafer, while restricting the travel of the stoichiometric metal compound being formed from the zone to thereby provide a stoichiometric ratio of sputtered metal atoms and reactive gas atoms adjacent the wafer to form the stoichiometric metal compound on the wafer.The target potential is raised or lowered by resetting the power level of a constant power source by feeding a target voltage monitor signal back to the constant power source as a power level set signal, while the reactive gas path is provided by a series of nested members which provide a path for the reactive gas while restricting the backflow of the stoichiometric metal compound being formed.